Conventional dryers typically only use hot air for drying. The material to be dried flows by gravity through the dryer. All of the heat for drying is provided by the hot air, which, as it picks up moisture, quickly cools and becomes saturated. The exhaust air typically is vented through a scrubbing system to atmosphere.
The disadvantage of this system is that it is very inefficient for energy utilization, typically achieving only about 30% in old fashioned dryers and between 30 to 50% efficiency in the best available technology. Where efficiency is measured in terms of: